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Nvidia announces G-Sync for laptops, reveals low-level tech details

G-Sync to make use of eDP on laptops, but desktops will still need a module.

Nvidia announces G-Sync for laptops, reveals low-level tech details
Nvidia

The big news from Nvidia might be the release of the GTX 980 Ti, but the company has also announced some updates for G-Sync. The variable refresh rate technology, which synchronizes the refresh rate of a compatible monitor to the frame rate of a game, is finally making the jump from desktop to laptop. Upcoming laptops from Gigabyte, MSI, Asus, and Clevo are all set to support the technology, and will feature 75Hz panels from 1080p all the way up to UHD (4K).

Desktop users aren't being forgotten either: There are some new monitors—including a lust-worthy 34-inch 21:9 75Hz IPS Acer X34 panel—featuring an updated G-Sync module that finally contains more than just a single DisplayPort input.

Interestingly, G-Sync for laptops makes use of the embedded DisplayPort (eDP) standard, a standardised interface for hooking up display panels directly to internal graphics cards. On the desktop, G-Sync can only be used with compatible monitors that contain Nvidia's G-Sync module.

According to Nvidia, the reason desktop displays need a G-Sync module is that it provides a much more controllable end-to-end solution for consistent performance. However, for G-Sync laptops, there's no module. Instead, the display is directly controlled by the GPU, which pulls double duty as both scaler and graphics card. G-Sync exploits this connection and the variable timing and panel self-refresh functionality built into eDP, effectively implementing G-Sync in software.

The more technically minded out there will note that this is very similar to how AMD's FreeSync works on the desktop, the tech being based DisplayPort Adaptive-Sync, which was in turn based on eDP.

While the underlying tech differs, Nvidia claims that G-Sync will be able to drive laptop displays in exactly the same way as it does in desktops, except by using standardised eDP features rather than any proprietary hardware. The end result should be the same: smooth gameplay without the input lag and judder of v-sync, or the screen tearing that occurs when v-sync is turned off.

Nvidia

While G-Sync on laptops is based on an open standard, not every laptop will bear the G-Sync compatible moniker. Naturally, the laptop will need an Nvidia GPU, but more importantly, the display has to be directly hooked up that Nvidia GPU. Nvidia's Optimus graphics solution, which allows the laptop to switch between an integrated GPU and the discrete GPU to save power, is not currently compatible with G-Sync. Nvidia is also implementing a qualification process for all manufacturers wishing to use the G-Sync brand, with individual components going through an optimisation process that looks at refresh properties and pixel response times in order to set the appropriate G-Sync timings.

But why does my desktop monitor need a G-Sync module...

The question is, if Nvidia can implement G-Sync on laptops using the open eDP standard, why can't it do the same thing with G-Sync on the desktop? It's an important question, given the company has decided to reveal some of the secret sauce behind G-Sync, particularly in comparison to FreeSync, and that all of it will work on laptops.

While FreeSync has had a positive reception, many reviewers (myself included) noted that there was some ghosting that wasn't present in G-Sync, as well as flickering issues at lower frame rates. The folks over at PC Perspective did an analysis of G-Sync and FreeSync a while back, looking at what happens at lower refresh rates, and it turns out their findings were nearly spot on about the underlying tech differences between G-Sync and FreeSync.

Nvidia has confirmed that the desktop versions of G-Sync, and the upcoming laptop version, make use of variable overdrive. The tech adjusts the amount of overdrive applied to a pixel—that is, driving pixels faster in order to get them to reach the desired colour sooner—in order to make them compatible with variable refresh timings. Overdrive was originally designed with a fixed refresh rate in mind. The display knows that if it pushes a pixel harder at exactly the right time, when the next refresh rolls around that pixel would gradually fade back down to the desired colour in a timely fashion, thus increasing the perceived response time.

Nvidia

Unfortunately, with variable response technology, the monitor no longer knows exactly when the next refresh window is going to be. In this situation, if overdrive continues to work in the same fashion, the pixels don't line up with the timing of the data coming from the graphics card, causing visible ghosting. According to Nvidia, this is what's happening with FreeSync, and G-Sync's variable overdrive tech solves it. Variable overdrive tries to predict when the next frame will arrive, and adjusts the timing of the overdrive to compensate. While this system will never be 100 percent accurate, causing a reduction in colour accuracy, it does help reduce ghosting, as evidenced by how well G-Sync works on the desktop.

In addition to variable overdrive, Nvidia also confirmed at a recent event what happens in a situation where the frame rate of a game drops below the minimum refresh rate of the monitor. Instead of reverting to v-sync on or off as FreeSync currently does, complete with its associated issues, G-Sync instead inserts duplicate frames. So, for example, while your system might be pushing a frame rate of 25 FPS, the G-Sync monitor actually doubles this to 50 FPS, pushing the game back into the variable refresh rate window.

Despite Nvidia's gentle prodding of AMD's FreeSync technology, the company did pick up a feature from its rival. G-Sync users now have the option of turning v-sync on or off when a game's frame rate rises above the maximum refresh rate of a monitor. While this isn't a particularly common occurrence if you're running a 144Hz TN panel, those with IPS panels that top out at 75Hz may find having the option useful. G-Sync now also works with games running in a window, which may be useful for those intent on getting in a sneaky bit of Dota 2 at the office, but won't settle for the judder-filled foibles of v-sync.

Listing image by Nvidia

Channel Ars Technica